Last and First Day of the previous months

--LAST DATE OF PREVIOUS MONTH

SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0))

--GET FIRST DATE OF PREVIOUS 3 MONTHS

SELECT @STARTDATE = DATEADD(DD,-(DAY(DATEADD(MM,1, GETDATE()))-1),DATEADD(MM,- 3, GETDATE()))